How Solar Cooling Works

Heat Collection

Collector plant selection is dependent on the required operating temperature of the chosen cooling process.

Air collectors are relatively low-temperature collectors used to directly heat air.  Air collectors are commercially available in Australia and have been particularly used for building space heating applications in cooler climates.  Air collectors have substantial cost reduction potential through building fabric integration and appear most suited to air-based cooling processes such solar desiccant cooling.

Flat plate collectors have been used with single stage absorption chillers even though collector efficiency is reduced at the temperatures commonly used for driving the chiller. Flat plate collectors are widely available, low-cost collectors used extensively for heating water in the residential and light commercial sectors.   

Evacuated tubes are used extensively in China and are increasingly appearing on the market in Australia. Evacuated tube collectors are non-tracking collectors capable of achieving significantly higher temperatures than that of flat plate collectors.

Tracking concentrating troughs reflect and focus direct-beam radiation onto a central receiver to achieve the highest temperatures and operate using oil, pressurised water or steam. The higher temperatures from trough collectors allow a more efficient two-stage absorption chiller to be used with a resulting reduction in the required area of the collector field.

With all collector technologies, designs should address the inevitable occurrence of much higher temperatures under stagnation conditions.  

Thermally-driven Cooling

Technologies used for converting solar heat into useful cooling include the following.

Desiccant cooling utilises liquid or solid desiccant material to dehumidify air.  After dehumidification the air is sufficiently dry to enable an evaporative cooling process to cool air well below ambient temperature conditions. This air is then supplied directly to the building. This is an open cycle process where the cooling process utilises water as the refrigerant and air as the delivery media.

While there are relatively few suppliers of these systems, desiccant cooling systems have been used extensively in certain niche applications (e.g. supermarkets) where the ability to independently control air humidity provides additional benefits.

Adsorption chillers perform a closed cycle batch adsorption/ desorption process using a refrigerant and a solid adsorbent to achieve refrigeration.  Refrigeration is used to cool down a secondary refrigerant circuit (chilled water or glycol) to enable the produced cold to be distributed to where it is required.

While there are only a limited number of Adsorption chiller manufacturers, adsorption chiller technology is able to operate with a lower temperature heat source and is more suitable for operation with a dry cooling tower.

Absorption Chillers use a liquid absorbent in a closed cycle process to achieve thermal compression of the refrigerant.  The resulting refrigeration process is used to cool down a secondary refrigerant circuit (chilled water or glycol) to enable the produced cold to be distributed to where it is required.

Absorption cooling technology is mature, low cost and supplied by numerous manufacturers with most commonly available chillers requiring a wet cooling tower.

Absorption chillers are more efficient than other thermal cooling processes which means that less solar heat is required to achieve a given amount of cooling.  Two-stage absorption chillers are even more efficient than single-stage units but require a higher temperature heat source.

Ejector refrigeration uses a thermal compressor (ejector) to compress a refrigerant without the use of any moving parts.  The technology is robust but to-date the technology has not been widely used due to its relatively low efficiency.

Typical heat source matches with cooling technology are summarised below.